In the normal frog and mammalian neuromuscular junction, in addition to the well described spontaneous miniature endplate potentials (MEPPs) there is a second class of spontaneous quanta one tenth the amplitude but with the same time characteristics as MEPPs. These sub-quanta are released from the nerve terminal but release characteristics are pharmacologically distinct from MEPPs, and sub-MEPP amplitudes are not changed with a variety of treatments that alter the MEPP size and amplitude distributions. The release of a single subunit generates a sub- MEPP, whereas the synchronous release of 10 subunits generates a MEPP. The possibility that sub-MEPPs and MEPPs are derived from different pools of transmitter will be investigated at the mouse diaphragm neuromuscular junction in vitro. The latter will be done with the aid of the false transmitter acetylmonoethylcholine (AMECh) and the acetylcholine synthesis blocker hemicholinium 3. The false transmitter will be incorporated into the terminal by increasing the rate of release with nerve stimulation or a high K+ medium. Release of AMECh will be identified by the shorter half decay time constant of the MEP currents recorded by a two electrode voltage clamp set-up. Differential effects of AMECh on the two classes of MEPPs would suggest that sub-MEPPs and MEPPs originate from different transmitter pools. In addition, the preparation will be incubated in hypertonic solutions which produce breaks on the rising phase of MEPPs. Since the amplitude of the breaks corresponds to the amplitude of a sub-MEPP, it appears that these atypical MEPPs result from a disruption in the subunit synchronizing mechanism. The analysis of the amplitudes of the break on MEPPs and the elapsed times between the start of MEPPs and the breaks will provide a quantitative description of co-operativity between the subunits of MEPP quanta and will thus define properties and constraints as to an organelle or a molecular basis of the subunit.